1. Field of the Invention
The present invention relates to optical add and drop multiplexers and de-multiplexers that can either remove or add a plurality of arbitrary channels selected form a larger set of wavelength-division multiplexed channels. More particularly, the present invention relates to such add and drop multiplexers that are re-configurable, such that the dropped and/or added wavelengths may be selectively changed during operation.
Description of the Related Art
In a wavelength division multiplexing optical communication system, information is carried by multiple channels, each with a distinct wavelength range. In this document, these individual information-carrying lights are referred to as either “signals” or “channels” and the totality of multiple combined signals, wherein each signal is of a different wavelength range, is referred to as a “composite optical signal.” Each one of the individual channels is identified by its representative or central wavelength, λi, where i is an index variable. Accordingly, a set of n channels is denoted generally denoted by the notation λ1-λn. A set of logically related channels is represented by the Greek letter, λ enclosed in brackets—for instance, a set of express channels is denoted by {λexp}, a set of dropped channels is denoted by {λdrop} and a set of added channels is denoted by {λ′}.
It is often necessary to add or drop a wavelength channel in optical links or networks. This can be achieved by a wavelength switch that sends different wavelength channels to different locations or pathways. If the pathways of the wavelengths can be selected or changed, such switches are re-configurable. Accordingly, a wavelength switch that extracts certain channels delivered from an input port and sends the selected extracted channels to a “drop” port and sends the remaining, or “express”, channels to another port, is a re-configurable de-multiplexer. Likewise, a wavelength switch that receives certain selected channels from an “add” port and that combines these channels together with express channels is a re-configurable multiplexer. A wavelength switch that performs both such drop and add operations as is a re-configurable add-drop multiplexer.
A conventional re-configurable de-multiplexer is illustrated in FIG. 12A. A conventional re-configurable multiplexer is generally constructed similarly to the apparatus illustrated in FIG. 12A, but with optical channels propagating in opposite directions to those shown. The prior-art re-configurable de-multiplexer 700 shown in FIG. 12 comprises an input port 702, a de-multiplexer 704, a plurality of n de-multiplexer output ports 706.1-706.n, a plurality of n 1×2 optical switches 708.1-708.n, wherein each such switch is optically coupled to a respective one of the multiplexer output ports, a plurality of n dropped-channel ports 710.1-710.n optically coupled to the 1×2 optical switches, a plurality of n express-channel ports 712.1-712.n optically coupled to the 1×2 optical switches, a multiplexer 714 optically coupled to the express channel ports and an output port 716.
The input port 702 comprising the conventional re-configurable de-multiplexer 700 delivers a plurality, n, of wavelength-division multiplexed channels, denoted λ1-λn, to the de-multiplexer 704, which separates the channels such that each channel is delivered to a different respective one of the de-multiplexer output ports 706.1-706.n. For instance, the de-multiplexer 704 delivers channel λ1 to the de-multiplexer output port 706.1, channel λ2 to the de-multiplexer output port 706.2, channel λ3 to the de-multiplexer output port 706.3 and channel λ1 to the de-multiplexer output port 706.n. The 1×2 switch 708.1 receives only the channel λ1 and can switch this channel λ1 to either the dropped-channel port 710.1 or to the express channel port 712.1. Likewise, each of the switches 708.2-708.n switches receives a different respective channel and switches it to either a dropped-channel port or to an express channel port. The multiplexer 714 receives channels from the express channel ports, re-combines these channels by wavelength-division multiplexing, and outputs the combined channels to the output port 716.
In the example shown in FIG. 12A, the switches 708.1 and 708.3 are shown in a configuration that sends the channels λ1 and λ3 to the dropped-channel ports 710.1 and 710.3, respectively and the switches 708.2 and 708.2 are shown in another configuration that sends the channels λ2 and λn to the express channel ports 712.2 and 712.n, respectively. Each one of the other switches (not shown) of the plurality of 1×2 switches may likewise be in either one of the same two configurations. Each switch can operate independently of all the remaining switches, so that any arbitrary set of channels may be dropped (i.e., delivered to a dropped-channel port), whilst the remaining express channels are re-multiplexed by the multiplexer 714 and output to the output port 716.
A conventional re-configurable add-drop multiplexer is illustrated in FIG. 12B. The conventional re-configurable add-drop multiplexer 750 shown in FIG. 12B is similar to the conventional de-multiplexer shown in FIG. 12A, except that provision is made to replace the dropped channels with replacement, or “added” channels, comprising the same respective wavelengths as the dropped channels. For clarity of presentation, only the possible pathways of three wavelengths λ1, λ2 and λ3 are shown in FIG. 12B, although a much larger number of wavelength-division multiplexed channels might travel through the apparatus 750. Since the conventional add-drop multiplexer 750 both adds and drops channels, it comprises a plurality of 2×2 optical switches 709.1-709.3, instead of the previously described 1×2 optical switches. The conventional add-drop multiplexer 750 also comprises a plurality of added channel ports 711.1-711.3, with each such added channel port optically coupled to a respective one of the 2×2 switches.
In the example shown in FIG. 12B, the switch 709.2 is shown in a configuration that sends the channel λ2 from the de-multiplexer output port 706.2 to the dropped-channel port 7102 and simultaneously sends the added channel λ′2 from the added channel port 711.2 to the express channel port 712.2. The switches 709.1 and 709.3 are shown in another configuration that sends the channels λ1 and λ3 to the express channel ports 712.1 and 712.3, respectively. Thus, the channel λ2 is dropped and replaced by the channel λ′2; the channels routed to the output port comprise the three channels λ1, λ′2 and λ3.
Although the conventional re-configurable de-multiplexer (FIG. 12A) and the conventional re-configurable add-drop multiplexer (FIG. 12B) can adequately perform their intended functions, it may be readily observed that, in order to maintain the flexibility of dropping (and, possibly, adding) anyone of the input channels, a separate 1×2 (or 2×2) switch is required for each one of the plurality of channels. Further, the conventional re-configurable de-multiplexer and conventional re-configurable add-drop multiplexer require provision of as many drop ports and as many add ports as the total number of possible input wavelengths. In practice, however, most network routing situations will only require a small subset of the original plurality of input channels to be dropped (and, possibly, replaced by added channels). Therefore, the requirement, in each of the described conventional apparatuses, for a separate switch corresponding to each channel adds unnecessary bulk, cost and complexity to these conventional apparatuses. There exists a need, therefore, for a re-configurable de-multiplexer, multiplexer or add/drop multiplexer that can add and/or drop multiple wavelengths and that retains the flexibility of adding or dropping any selected wavelength without requiring a separate switch for each wavelength. The present invention addresses this need.